(CNN) -- Twelve American ecotourists who went missing Monday in China's earthquake used a satellite phone to let their loved ones know they are alive and well, an official said Wednesday.

American tourists were visiting the the Wolong Giant Panda Reserve when the earthquake hit Monday.

But two Chinese volunteers with the World Wildlife Fund, which sponsored the tour, remained missing, a spokesperson for the organization said Wednesday.

"All 12 tourists are safe and accounted for. They wanted us to know that they were safe and to let their families that they were OK," said Kerry Zobor, spokesperson for the WWF.

"While we are relieved by this positive news about our travel group, we remain deeply concerned about missing volunteers," WWF officials said in a statement released Wednesday. The missing volunteers were not on the tour. They were last seen in in Maoxian County, according to the statement.

The tour group, which consisted of World Wildlife Fund members, was visiting the breeding center at Wolong Giant Panda Reserve in Sichuan province when the deadly earthquake struck Monday. To date, authorities have put the death toll at 15,000.

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Since the quake, the WWF has been scrambling to establish contact with the group, which began its tour in Beijing on May 5 and was scheduled to depart from Shanghai on May 18.

Shattered infrastructure in the region hampered those efforts until Wednesday, when the WWF's office in China was able to make contact with officials at the reserve using a satellite telephone.

Satellite phones are a common tool for researchers and volunteers with the WWF in Wolong, where telecommunications are spotty at best, Zobor said.

The WWF said it had notified the relatives of the tourists, whose names are being withheld by the organization.

Zobor said the group is still on the Wolong Giant Panda Reserve, one of China's preeminent giant panda research and breeding facilities. On Tuesday, China's official news agency, Xinhua, confirmed the safety of the 86 pandas inhabiting the grounds.